1. Field of the Invention
The invention relates generally to methods and apparatus for drilling and completing well bores. More specifically, the invention relates to methods and apparatus for a drillable bridge plug.
2. Background Art
In drilling, completing, or reworking wells, it often becomes necessary to isolate particular zones within the well. In some applications, downhole tools, known as temporary or permanent bridge plugs, are inserted into the well to isolate zones. The purpose of the bridge plugs is to isolate some portion of the well from another portion of the well. In some instances, perforations in the well in one section need to be isolated from perforations in another section of the well. In other situations, there may be a need to use a bridge plug to isolate the bottom of the well from the wellhead.
When it is desired to remove one or more of these bridge plugs from a wellbore, it is often simpler and less expensive to mill or drill them out rather than to implement a complex retrieving operation. In milling, a milling cutter is used to grind the tool, or at least the outer components thereof, out of the well bore. In drilling, a drill bit or mill is used to cut and grind up the components of the bridge plug to remove it from the wellbore.
Drillable bridge plugs are typically constructed of a metal such as cast iron that can be drilled out. One typical problem with conventional drillable bridge plugs is that cast iron is difficult to drill out. This may result in extremely long drill-out times, excessive casing wear, or both. Long drill-out times are highly undesirable as rig time is typically charged by the hour. Additionally, the mandrel often falls out of the backup rings and slip assemblies once a single key locking the upper assembly and the mandrel is drilled out. The falling mandrel may damage other components below the plug in the well. Another typical problem with conventional drillable plugs is that cast iron plugs are generally required to be robust to achieve an isolating seal. These typical plugs, thus require that significant weight be applied to the drill bit in order to drill the plug out. It would be desirable to create a plug that did not require significant forces to be applied to the drill bit such that the drilling operation could be accomplished with a coiled tubing motor and bit.
Some embodiments of drillable plugs known in the prior art solve this problem by providing an apparatus wherein at least some of the components, including pressure bearing components, are made of non-metallic materials, such as engineering grade plastics, or composite materials. Examples of a composite bridge plug may be found in U.S. Pat. No. 6,796,376 B2, issued to Frazier, and is incorporated by reference in its entirety. Such plastic or composite components are much more easily drilled than cast iron. However, when several plugs are used in succession to isolate a plurality of zones within the well bore, there may be significant heat and pressure on the plug from either side. Plugs with pressure bearing components made of non-metallic material tend to fail at high temperature and pressure. Composite materials may have a faster drill out, but the reliability is lower due to a tendency to delaminate. The fabrics from composite materials tend to string out, or ball up, thus plugging up the production line. Additionally, the fabrics, because of their low mass, often float up with the gas into the production line, possibly causing plugs or failure of the production lines.
In other embodiments known in the prior art, some components of the plug are made of aluminum alloys. Plugs made of aircraft quality aluminum alloys in the prior art, when drilled, often result in a “gummy” material, wherein the material “balls” during milling or drilling. That is, the material melts and then cools during the drilling process. As the aluminum material of the prior art plugs melts and cools during drilling, it adheres to the cutting structure of the drill, thereby making the drilling process less efficient.